The present invention relates generally to a technique for pressurizing and depressurizing a pressurizable container or housing and more particularly to a device which is capable of pressurizing or depressurizing the container or housing in a linear, fixed manner. As will be seen hereinafter, this device is especially suitable for use in an arrangement for taking blood pressure in which a cuff wrapped around the arm or other suitable body part of a patient is first pressurized to a given level and then depressurized sufficient to detect the patient's systolic and diastolic pressures.
Turning briefly to the drawings and specifically FIG. 1, a conventional arrangement for taking blood pressure of a given patient is diagrammatically illustrated and designed by the reference numeral 10. This arrangement includes a standard pressurizable cuff 12 wrapped around the arm 13 (or other suitable body part) of a patient and means generally indicated at 14 for first pressurizing the cuff to a given level above the patient's anticipated systolic pressure and then depressurizing it to a level below the patient's anticipated diastolic pressure. While not shown, the overall arrangement includes suitable means for actually detecting the patient's systolic and diastolic pressures as the cuff is depressurized. For example, in the case of those arrangements which utilized Korotkoff sounds, a stethoscope or a functionally equivalent electronic means may be provided.
In the conventional arrangement described immediately above, once the cuff is pressurized to the desired level and then caused to depressurize, it typically does so in a non-linear, somewhat exponential fashion, as illustrated best in FIG. 2. This Figure graphically illustrates the depressurization of cuff 12 with time for three different sizes of cuffs, a medium size cuff A, a large cuff B and a small cuff C. In each case, the rate of depressurization is greatest in the early stages and then slows down as the pressure within the cuff decreases. As a result, it takes substantially longer to measure the patient's systolic and diastolic pressures then would be case if the cuff could be made to depressurize linearly. Take for example the depressurization ramp (curve) A for a medium cuff. The cuff is initially pressurized to a level above the patients systolic pressure and as it depressurizes, it reaches the systolic pressure S at the time t1 and then the patient's diastolic pressure at the time t3. If the cuff could be made to depressurize linearly, as indicated by the linear ramp (curve) A' the patient's diastolic pressure D would be measured at time t2, substantially earlier than time t3. This time differential between t3 and t2 for standard size arm cuffs is on the order of approximately 10 seconds.
Because arrangement 10 does not depressurize its cuff in a linear fashion but rather exponentially (as described above), it has the disadvantage of being slower than it would be if the cuff could be linearly depressurized. Another disadvantage of the arrangement illustrated relates to the size of cuff 12. Specifically, cuffs of different sizes result in depressurization ramps of different configurations. As illustrated in FIG. 2, the ramp A corresponds to a medium size cuff, the ramp B corresponds to a larger cuff, and the ramp C corresponds to a smaller cuff. Thus, if arrangement 10 includes an electronic means for detecting the patient's systolic and diastolic pressures and if the arrangement uses cuffs of different sizes, suitable circuitry and an adjustment switch would be required to compensate for the different sized cuffs to make the ramps for all cuff sizes approximately equal.